mm: Define VM_SHADOW_STACK for arm64 when we support GCS

Use VM_HIGH_ARCH_5 for guarded control stack pages.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-14-222b78d87eee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/filesystems/proc.rst

@@ -579,7 +579,7 @@ encoded manner. The codes are the following:
     mt    arm64 MTE allocation tags are enabled
     um    userfaultfd missing tracking
     uw    userfaultfd wr-protect tracking
-    ss    shadow stack page
+    ss    shadow/guarded control stack page
     sl    sealed
     ==    =======================================
 

include/linux/mm.h

@@ -367,7 +367,17 @@ extern unsigned int kobjsize(const void *objp);
  * for more details on the guard size.
  */
 # define VM_SHADOW_STACK	VM_HIGH_ARCH_5
-#else
+#endif
+
+#if defined(CONFIG_ARM64_GCS)
+/*
+ * arm64's Guarded Control Stack implements similar functionality and
+ * has similar constraints to shadow stacks.
+ */
+# define VM_SHADOW_STACK	VM_HIGH_ARCH_6
+#endif
+
+#ifndef VM_SHADOW_STACK
 # define VM_SHADOW_STACK	VM_NONE
 #endif